Imidazolate-bridge Cu

The imidazolate bridged Cu/Zn bimetallic complex of the cryptand (13) was structurally characterized and shown to have a Cu-Zn distance of 5.93 A (native Cu, Zn-SOD 6.2 A).146 The complex shows some activity in the dismutation of superoxide at biological pH that is retained in the presence of bovine serum albumin. 

The ESR spectrum of the Cu —Zn heterodinuclear complex gave the well-defined ESR parameters (gy = 2.10, gJ. = 2.24,14 l = 117 G, and IA LI = 124 G), which indicate that the Cu(ll) ion in the complex has a trigonal-bipyramidal environment and a ground state (154). The observation of well-defined ESR spectmm also confirms that the complex retains its imidazolate-bridged Cu —Zn heterodinuclear structure and that it is not a 1 1 mixture of Cu —Cu and Zn Zn homodinuclear complexes. 

Scheme 25. The catalytic cycle of the imidazolate-bridged Cu —heterodinuclear SOD model complex (154).

The same mechanism as Scheme 25 can be applied to the disproportionation of semiquinone radical anion (Q ) by the imidazolate-bridged Cu —Zn complex (155), (Scheme 25). Since the one-electron oxidation potential (Lqx vs SCE) of Q (—0.51 V), which is equal to the one-electron reduction potential of Q (135), is more negative than the one-electron reduction potential ( red vs SCE) of the Cu(II)... 

Imidazoles are of interest as bridging ligands particularly with regard to mimics of the active site of Cu-Zn superoxide dismutase (SOD). Structures with imidazolate bridges have been... 

The imidazole-bridge dimetallic centre in copper-zinc superoxide dismutase (EC 1.15.1.1) was a novel structural feature that had not previously been encountered in coordination chemistry [151], The Cu(II) ion is co-ordinated by four histidine side chains, His44, His46, His 118 and His61, and there is evidence for a fifth axial water ligand. 

The structure and enzyme kinetics of bovine erythrocyte superoxide dismutase are reviewed. The protein has a novel imidazolate-bridged copper(II)-zinc(II) catalytic center in each of two identical subunits. Since a C /Cu1 redox couple is responsible for the dismutase activity of the enzyme, the role of zinc is of interest. Both 220-MHz NMR measurements of the exchangeable histidine protons and chemical modifications using diethylpyrocarbonate demonstrate that zinc alone can fold the protein chain in the region of the active site into a conformation resembling that of the native enzyme. Other possible roles for zinc are discussed. Synthetic, magnetic, and structural studies of soluble, imidazolate-bridged copper complexes of relevance to the 4 Cu(II) form of the enzyme have been made. 

As mentioned in Section IVA, mixed ligand complexes of formula [([12]aneN3)M2(L8)] (where M is Zn or Cu) have been synthesized and characterized spectroscopically (107). The data are consistent with an imidazolate bridged structure as illustrated in Fig. 33. EPR spectroscopy on the dicopper(I) complex shows evidence of coupling there is a weak AM = 2 transition at half field (ca. 1560 G compared with ca. 1540 G in CU2CU2SOD). Attempts to prepare heterodinuclear CuZn complexes as Cu2Zn2SOD models have, so far, been unsuccessful. However, some heterodinuclear complexes of related ligands have been... 

Amino-l-ribosylimidazole-4-carboxamide, cyclization of 78JHC353. Imidazole analogues of nucleic acid components 84MI5. Imidazole-bridged bimetallic center in Cu Zn superoxide dismutase and its... 

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